A surveillance image released by the FBI shows Aaron Alexis in the hallway of building #197 carrying a Remington shotgun. / FBI

by Kevin Johnson and Doug Stanglin, USA TODAY

by Kevin Johnson and Doug Stanglin, USA TODAY

WASHINGTON -- An increasingly delusional Aaron Alexis believed he was being controlled by low-frequency electromagnetic waves over a period of months, driving him last week to indiscriminately fire on employees at the Navy Yard, federal authorities said Wednesday.

Federal officials also released chilling excerpts of surveillance video showing Alexis, 34, armed with a sawed-off shotgun patrolling the halls and a stairwell of Building 197 at the naval complex where he killed 12 and wounded four others.

In one segment, Alexis seeks cover behind a hallway corner like a soldier preparing for room-to-room combat.

In the fullest explanation yet of what prompted the Sept. 16 massacre, Assistant FBI Director Valerie Parlave referred to communications recovered from Alexis' laptop and other electronic devices that indicated the military contractor believed he had been "targeted'' by the low-frequency waves.

"Ultra-low frequency attack is what I've been subject to for the last three months,'' Alexis wrote in one message, "and to be perfectly honest that is what has driven me to this.''

A reference to his obsession with the electromagnetic waves was found among a series of scrawled etchings on the 870 Remington shotgun used in the attack, one of which stated: "My ELF weapon.''

Other scrawled messages on the gun included: "End to the torment!'' "Not what ya'll say.'' "Better off this way!''

Parlave said "ELF'' is an apparent reference to the extremely low-frequency waves.

The assistant director said the so-called "ELF'' technology was a legitimate program for sub-tonal submarine communications. However, conspiracy theories suggest that the technology is used for government monitoring and manipulation of unsuspecting people.

"There are multiple indicators that Alexis held a delusional belief that he was being controlled or influenced by low-frequency electromagnetic waves,'' Parlave said.

One of those indicators emerged last month when police were called to a Newport, R.I., hotel room where a distressed Alexis told officers that he was hearing voices through the floor and ceiling of his room.

An analysis of Alexis' writings, which Parlave said were not apparently shared with family or friends, also indicate that Alexis viewed the assault as a likely suicide mission.

"There are indications that Alexis was prepared to die during the attack,'' Parlave said, "and that he accepted death as the inevitable consequence of his actions.''

In all of the evidence gathered so far, the assistant director said there is nothing to indicate that Alexis, who acted alone, was "targeting specific individuals.''

A fuller timeline of the Sept. 16 events show that more than an hour had elapsed from the time Alexis shot his first victim at 8:16 a.m. in the "4 West'' area of the building until the gunman was killed by police at 9:25 a.m.

As a private contractor assigned to a information technology project at the Navy Yard, authorities maintain that Alexis had legitimate access to the property when at 7:53 a.m., he entered a Navy Yard parking garage driving his rental Toyota Prius with New York license plates.

At 8:08 a.m., according to the FBI's chronology, Alexis entered the building with a valid pass and proceeded to the fourth-floor restroom carrying a backpack and a clipboard.

It is believed that Alexis assembled the weapon in the restroom before emerging with the fully assembled shotgun which he brandished in the surveillance video. He later obtained a Beretta handgun during the course of the attack.

Within a minute after the first victim was shot, authorities reported receiving the first call for help from the building's fourth floor.

What followed, according to the chronology, is a chaotic sequence in which Alexis descended to the third floor, then "appeared'' on the first floor before he returned to the third level where he was shot to death by police.

Among the items found near his body were the loaded handgun, multiple expended cartridge casings and shotgun shells.

The backpack used to conceal the weapon was recovered in the fourth-floor restroom hanging on the back of a stall door.

Inside, investigators later found thumb drives and other electronic data storage devices that continue to be analyzed by authorities.

Noting other recent mass shootings that have scarred Tucson, Ariz., Aurora, Colo., and Newtown, Conn., U.S. Attorney Ronald Machen Jr., said the incident marked yet another case in which "a mentally unstable individual was allowed to obtain a firearm in an attack designed to kill as many individuals as possible.''

In the Navy Yard case, Alexis' purchase of the shotgun, just two days before the attack, was legal. Though a series of past incidents suggested mental illness, he was never legally declared mentally ill by a court or convicted of a felony, which would have blocked his Sept. 14 gun purchase.